In power door lock operations, a drive motor is utilized to reciprocally drive or shift a lift arm that is connected to a locking lever of a door latch assembly mounted in an automobile door. The lift arm is typically coupled to an output shaft of the drive motor via an intermediate gear train and operates to position the locking lever in either a locked or an unlocked position.
Additionally, the lift arm can be manually driven or shifted by either repositioning a door lock knob or slider, or by use of a door key. Since the gear train and output shaft are directly coupled to the lift arm, manually shifting the lift arm into the locked position requires driving the gear train and the output shaft, and shifting the lift arm into the unlocked position requires backdriving the gear train and the output shaft. In both cases, the drive motor and gear train undesirably offer resistance to being manually driven/backdriven by the door key, or by repositioning the door lock slider. Relatively speaking, the drive motor offers substantially greater resistance to being manually driven/backdriven than the gear train.
The ease with which a lift arm can be manually shifted by use of a door key or door lock slider is referred to as the key effort or reversibility of the power door lock system which is a measure of the amount of resistance provided by the drive motor and gear train when the lift arm is manually shifted. Thus, the greater resistance provided by the gear train and the drive motor, the greater the key effort required to shift the lift arm and the higher the reversibility of the power door lock system.
One solution to the problem of driving/backdriving the motor during manual operation is by use of a clutch interposed between the output shaft of the drive motor and the gear train. The clutch operates to selectively mechanically couple the output shaft of the motor to the lift arm when the motor is activated, such as during a power door lock or unlock operation, and to decouple the output shaft from the lift arm when the motor is deactivated to thereby permit manual shifting of the lift arm without additionally driving/backdriving the motor.
Thus, when the clutch decouples the motor from the lift arm, the key effort required to unlock the car door with a key, or by repositioning the door lock slider is desirably reduced. The key effort is reduced because only the lift arm, jack screw and gear train are driven/backdriven without additionally driving/backdriving the motor.
Centrifugal clutches for use in selectively establishing a mechanical driving connection between the output shaft and the lift arm are known. For example, EP 0 233 820 discloses a centrifugal clutch interposed between a drive motor output shaft and a gear train of a power door lock actuator. The disclosed centrifugal clutch includes numerous parts such as springs which complicate the clutch design, reduce operational reliability of the clutch, complicate the manufacturing and assembly process, and ultimately increase manufacturing and maintenance costs.
What is needed therefore is a simplified centrifugal clutch design which reduces the number of parts for assembly and thereby increases reliability and reduces labor and material costs.